The identification of a chromosomal biomarker for high LET radiation would facilitate the identification of individuals exposed not only to domestic and occupational radon which may be responsible for specific health effects, but also to fission neutrons. Studies have suggested that high LET radiation produces a low ratio [F] of inter- to intrachromosome interarm exchange aberrations compared with low LET radiation. Using multicolor fluorescence in situ hybridization [FISH] painting we analyzed the frequency of inter- and intrachromosome exchanges.Normal human fibroblasts in the G1 phase and G0 human lymphocytes were exposed to 2-8 Gy of 0.6 Gy /hr 137 Cs gamma -rays , 0.2-0.8 Gy of 430 keV neutrons , or 0.2-0.8 Gy 65 keV neutrons. The F-ratio was found to be dose dependent and confounded by the high proportion of complex exchanges observed after high LET radiation. A higher frequency of complex exchanges was observed after neutron radiation [22-30%] compared with LDR gamma -rays [8-22%]. The frequency of complex exchanges was found to be higher in lymphocytes than fibroblasts , particularly after high LET radiation. These studies encourage consideration of this approach for the analysis of latent viable chromosomal changes.